A number of patents and publications are cited herein in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Each of these references is incorporated herein by reference in its entirety into the present disclosure, to the same extent as if each individual reference was specifically and individually indicated to be incorporated by reference.
Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise,” and variations such as “comprises” and “comprising,” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.
It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a pharmaceutical carrier” includes mixtures of two or more such carriers, and the like.
Ranges are often expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment.
This disclosure includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
Axl receptor tyrosine kinase (nucleotide accession numbers NM—021913 and NM—001699) is a transmembrane receptor tyrosine kinase (RTK) protein and a member of the Axl RTK subfamily. The Axl RTK gene has a chromosomal location of 19q13.2 (see, e.g., O'Bryan et al., 1991). Axl is also known as UFO, ARK and TYRO7 and has IUBMB Enzyme Nomenclature EC 2.7.10.1.
The Axl RTK subfamily comprises Axl, Mer (Stk, Nyk) and Tyro3(Rse/Dtk/Sky). This subfamily is characterised by a common protein domain structure. The Axl RTK subfamily all possess the combination of two extracellular N-terminal immunoglobulin type domains and two fibronectin III domains, a single span transmembrane region followed by C-terminal kinase domain (see, e.g., Hafizi et al., 2006a).
Gas6 acts as a ligand to all of the Axl RTK family, but exhibits differing affinities for the receptors and activates the three proteins to varying degrees. Gas6 is a member of the vitamin K-dependent family of proteins and shows a 43% sequence identity and the same domain organisation to protein S, a serum protein which has been shown to be a negative regulator of blood coagulation (see, e.g., Hafizi et al., 2006b).
Gas6 is upregulated in growth arrested cells (see, e.g., Manfioletti et al., 1993) which indicates a function in protection of the cell against cellular stresses. It has since been shown that Gas6 can cross link Axl monomers and promote cellular survival, proliferation and migration (see, e.g., Bellosta et al., 1997; Sainaghi et al., 2005; Fridell et al., 1998).
Homophilic binding of the Axl extracellular domain can result in cellular aggregation and this event is independent of the intracellular kinase activity (see, e.g., Bellosta et al., 1995).
The Axl intracellular kinase domain (ICD) is responsible for the oncogenic transforming ability of Axl RTK. The Gas6/Axl signal transduction pathway operates, although not exclusively, through activation of the phosphatidylinositol 3-kinase (PI3K) pathway (see, e.g., Shankar et al., 2006).
The PI3K/Akt signaling network is crucial to widely divergent physiological processes that include cell cycle progression, differentiation, transcription, translation and apoptosis (see, e.g., Hanada et al., 2004).
Activation of PI3K/Akt signaling results in disturbance of control of cell proliferation and apoptosis, ensuing in competitive growth advantage for tumor cells. Activation of Akt is associated with phosphorylation of Ser473 (see, e.g., Alessi et al., 1996) and monitoring changes in levels of total and phosphorolated Akt within the cell enables an assessment of the efficacy of drugs which act upstream of Akt.
The intracellular domain of Axl kinase has been shown to associate with many proteins (p55gamma, p85alpha and beta subunits of PI3K, phospholipaseC-gamma, Grb2, c-Src, Lck, SOCS-1, Nck2, RanBMP, C1-TEN and Axl ICD itself) (see, e.g., Hafizi et al., 2006a; Braunger et al., 1997; Hafizi et al., 2002).
Axl is ubiquitously expressed at low levels and is detectable in a variety of organs (see, e.g., Rescigno et al., 1991). Expression patterns of the other two family members (Mer and Tyro3) differ from that of Axl. Expression of Tyro3 is predominantly in the brain and the CNS (see, e.g., Mark et al., 1994), and expression of Mer is almost exclusively in the monocyte cell lineage (see, e.g., Graham et al., 1994).
Overexpression of Axl has been demonstrated in numerous cancer cell lines (e.g., colon, gastric, breast, lung, AML, thyroid, ocular, prostate, ocular melanoma, ovarian, renal, and SCC) (see, e.g., Sainaghi et al., 2005; Sawaby et al., 2007; Vajkoczy et al., 2006; Meric et al., 2002; Shieh et al., 2005). This expression has been linked to the development of oncogenic cellular phenotype (see, e.g., Shieh et al., 2005). Overexpression of Axl has been linked to stage of disease and prognosis (see, e.g., Sawabu et al., 2007; Shieh et al., 2005; Sun et al., 2003; Green et al., 2006).
Modulation of Axl levels in vivo and in vitro demonstrates the involvement of Axl in the progression of a cancer phenotype. siRNA mediated silencing demonstrates that Axl is a regulator of multiple angiogenic behaviours in vitro and Axl knockdown reduces growth of breast carcinoma cell lines in a xenograft (see, e.g., Holland et al., 2005).
There is a need for additional and better therapeutic agents for the treatment of proliferative conditions, such as cancer, etc., including, for example, additional and better therapeutic agents that inhibit AXL receptor tyrosine kinase function.